CN111505066A - A three-dimensional capacitance tomography device for flow in a cryogenic fluid venturi tube - Google Patents

Abstract

The invention discloses a three-dimensional capacitance tomography device applied to flow monitoring in a deep cryogenic fluid (such as liquid nitrogen (78K) and liquid oxygen (90K)) Venturi tube. The device comprises: conductor rod, metal casing, venturi, electrode piece cover, electrode slice, annular connection shell. The device sets up electrode slice and the electrode slice cover rather than the surperficial laminating mutually to venturi variable diameter's characteristics, designs corresponding wire connection structure simultaneously and designs shielding shell to the characteristics that electric capacity tomography imaging needs the shielding. The three-dimensional capacitance tomography device of the low-temperature Venturi tube has the following characteristics: the structure is simple and stable, the disassembly and the assembly are simple, the disassembly and the assembly can be carried out for many times, and the pipeline can be quickly connected with other pipelines; can be used in a wide range from room temperature to deep low temperature, and has good electromagnetic shielding effect.

Description

A three-dimensional capacitance tomography device for flow in a cryogenic fluid venturi tube

technical field

The invention relates to the field of low-temperature refrigeration engineering technology and sensor science and technology, in particular to a three-dimensional capacitance tomography (ECVT) device for flow in a low-temperature fluid venturi tube.

Background technique

Two-phase flow in the tube is very common in the industrial production process. Measuring the distribution of the two-phase flow in the tube is of great significance to study the heat transfer and flow mechanism of the two-phase flow. optimization. In the pipeline structure, the type of variable-diameter pipeline such as Venturi is a common situation. This type of pipeline has a complex structure and needs special attention. Three-dimensional capacitance tomography (Electrical Capacitance Volume Tomography, ECVT) technology uses the spatial difference of dielectric constant caused by different phase distribution in the tube to measure the capacitance value between multiple pole pieces to determine the spatial phase of the fluid in the tube. It is a non-invasive measurement technology that has the advantages of responding to the velocity block, no interference to the flow field, and can simultaneously obtain the phase holdup and phase distribution data. It has been widely studied and obtained in the normal temperature fluid industry. application.

Compared with the two-phase flow in the tube at room temperature, the liquid-gas permittivity ratio of cryogenic fluids is nearly an order of magnitude smaller, usually within 1.5. Therefore, capacitance tomography at low temperature is very sensitive to external disturbances in the measurement process, while three-dimensional capacitance tomography is more sensitive to noise than two-dimensional, so a good shielding device should be used in sensor design. The varying diameter of the pipes makes the installation of the pole pieces more difficult compared to conventional 3D capacitance tomography sensors, thus requiring the design of a new structure. The invention is designed according to the phase distribution requirements of the low-temperature two-phase flow, synthesizing the requirements of the low-temperature fluid for shielding and the requirements of the variable-diameter pipeline for the installation of the pole pieces.

Zhang Xiaobin, Chen Jianye, Wang Yuchen, Xie Huangjun, Institute of Refrigeration and Cryogenics, Zhejiang University ^[1][2][3] and others have successively studied the diode capacitive sensor used in the measurement of the cavitation rate of the two-phase flow of cryogenic fluids and its application in cryogenic fluids. The theoretical and experimental researches were carried out on the capacitance tomography technique for the measurement of distribution and cavitation rate. Chen Jianye's experiments show that the measurement error of the diode capacitance sensor for the cavitation rate of the low-temperature fluid in the tube can be controlled within 15%, but the sensor is wrapped with two electrode pieces and does not adopt an electromagnetic shielding structure, which is different from the structure of the present invention. The difference is huge. Xie Huangjun conducted a theoretical verification of the capacitance tomography technology applied to the measurement of low temperature fluids, and found that the results of its numerical experiments were good, but the article did not involve the specific structure design of the capacitance tomography sensor for the measurement of low temperature fluid two-phase flow.

Chinese patent CN108152341A discloses a flow capacitance tomography device in a low-temperature fluid tube. The invention aims to provide a tightening stud for the shrinkage of the material at low temperature to maintain the fit of the electrode sheet and the insulating tube, and to adjust the liquid-gas dielectric constant of the low-temperature fluid. The shielding structure is designed with similar characteristics, which is quite different from the present invention in structure.

Chinese patent CN106370705A discloses a three-dimensional capacitance tomography sensor. The diameter of the pipe used in the invention remains unchanged, and there is no wire path connected to the pole piece, which is quite different from the invention in structure.

US Patent US2017/0328853A1 discloses a smart sensor for three-dimensional capacitance tomography. The invention installs the electrode sheet in a structure with a variable diameter along the radial direction, the diameter is not variable along the axial direction, and no shielding structure is provided , which is quite different from the present invention in structure.

To sum up, according to the characteristics of low-temperature fluid gas-liquid phase dielectric constant and the characteristics of variable-diameter pipes, it is necessary to design a three-dimensional capacitance tomography device specially designed for the flow of low-temperature fluids in variable-diameter pipes. The characteristics of non-invasive measurement can obtain the distribution of phase content in the pipeline in three-dimensional space.

[1] CHEN J Y, WANG Y C, ZHANG W, et al. Capacity-based liquid holdup measurement of cryogenic two-phase flow in a nearly-horizontal tube. Cryogenics 2017;84:69-75.

[2] Wang Yuchen, Chen Jianye, Xu Lu, et al. Measurement method of liquid film thickness of low temperature fluid in tube based on capacitance method [J]. Journal of Zhejiang University: Engineering Edition, 2016, 50(10): 1855-1858.

[3] Xie H, Yu L, Zhou R, et al. Preliminary evaluation of cryogenic two-phase flow imaging using electrical capacitance tomography [J]. Cryogenics, 2017, 86.

SUMMARY OF THE INVENTION

The invention provides a three-dimensional capacitance tomography (ECVT) device flowing in a low-temperature fluid venturi tube. The device can reflect the low temperature in the venturi tube in the form of an image by measuring the capacitance between two electrode sheets and processing the capacitance data accordingly. Fluid two-phase flow phase distribution. The device has the characteristics of non-invasive, simple and stable structure, simple disassembly and assembly, can be disassembled and assembled many times, and can be quickly connected with other pipelines; it can be used in a wide range from room temperature to deep and low temperature, and the electromagnetic shielding effect is good.

The technical scheme of the present invention is as follows:

The invention discloses a low-temperature venturi tube three-dimensional capacitance tomography (ECVT) device, which comprises the following components: a conductor rod, a metal casing, a venturi tube, an electrode sheet cover, an electrode sheet, and an annular connection shell. The electrode piece is embedded in the electrode piece cover, the electrode piece cover wraps the venturi tube, the annular connecting shell wraps the electrode piece cover, the metal shell wraps the annular connecting shell, and the conductor rod is inserted into the metal shell to contact the metal hemisphere on the annular connecting shell.

Further, the electrode sheet has a plurality of pieces, which are curved surfaces whose outer surface area is larger than the inner surface area, and are closely attached to the corresponding electrode sheet grooves on the electrode sheet sleeve. The electrode sheet sleeve is provided with a wire groove near the upper part of the electrode sheet groove and extends to the end of the electrode sheet sleeve. The inner surface of the electrode sleeve embedded in the electrode plate completely coincides with the outer surface of the Venturi tube.

Further, the electrode sheet sleeve, whose axial length is the same as that of the venturi tube, is provided with electrode sheet grooves staggered along the axial direction. There are connecting bosses and connecting grooves at both ends of the electrode sleeve, and the end of the connecting boss has barbs. When two identical electrode sleeves are connected, the barbs of the connecting bosses are inserted into the connecting grooves to connect and fit the Venturi tube. . The part of the electrode sheet sleeve close to the connection boss is also provided with a boss for fixing the annular connection shell.

Further, the outer diameter of the annular connection shell is the same as the largest diameter at both ends of the electrode sheet sleeve. The two ends have an inclination angle along the axial direction, which is the same as the inclination angle of the outer surface of the electrode sleeve. The annular connecting shell is provided with a positioning groove matched with the positioning boss on the electrode sheet sleeve, two identical annular connecting shells surround the electrode sheet sleeve, and the positioning groove is closely fitted with the positioning boss on the electrode sheet. The annular connection shell is provided with a wire hole connected to the wire groove at the end of the electrode sheet sleeve near the end, and the wire hole is communicated with the wire groove on the outer surface thereof. The outer surface of the annular connection shell is provided with a metal conductor hemisphere, and the metal hemisphere is provided with screw holes, which can be fixed on the outer surface of the annular connection shell by passing through the hemisphere with screws. The metal hemisphere is connected to the wire slot.

Further, the inside of the metal shell is provided with semi-cylindrical grooves corresponding to the number of metal hemispheres on the annular connecting shell, and the radius of the cylinder is the same as that of the metal hemispheres. The inner diameter of the metal shell is equal to the outer diameter of the annular connecting shell, and the axial length is slightly larger than that of the Venturi tube. At the same time, the metal shell is provided with through holes with the same number and corresponding positions as the metal hemispheres on the annular connecting shell, and the hole diameter is slightly larger than the outer diameter of the metal hemispheres. The metal shell is sleeved into the annular connecting shell, the axis of the through hole is aligned with the spherical center of the metal hemisphere, there is a certain gap between the metal hemisphere and the metal shell, and the two do not contact.

Further, the number of the conductor rods is the same as that of the through holes on the metal shell, the outer diameter is equal to the through holes, the insertion through holes are in contact with the metal hemisphere on the annular connection shell, and the elastic metal sheet at the end of the conductor rod has a certain amount of contact with the metal hemisphere. Elastic deformation to ensure full contact with the metal hemisphere.

Compared with the prior art, the present invention adopts an integrated electrode sheet sleeve, which is attached to the measuring pipe, which simplifies the fixing and installation of a plurality of electrode sheets on the variable-diameter pipe into a simple embedding work, and overcomes the problem of The difficulty of installation on the variable-diameter pipe makes the installation of the electrode sheet on the non-uniform pipe simple and fast, and the relative position of the electrode sheet and the pipe can be kept unchanged during the measurement process. At the same time, the metal casing is used as a connection and shielding device with other pipes and is matched with the corresponding line connection structure, which overcomes the difficulty of shielding interference devices required by traditional capacitance tomography and these shielding devices will introduce complex structure line structures. The circuit layout of the analysis image is simplified, which is convenient for disassembly and assembly. In addition, the present invention adopts a line structure combining wire grooves and conductor rods, which overcomes the difficulty of interfering with the measured value due to vibration changes in the relative positions of different lines during the measurement process, and makes the measurement result closer to the real value.

Description of drawings

FIG. 1 is a front view of a three-dimensional capacitance tomography (ECVT) device for flow in a cryogenic fluid venturi tube according to the present invention.

FIG. 2 is a left side view of a three-dimensional capacitance tomography (ECVT) device for flow in a cryogenic fluid venturi tube according to the present invention.

FIG. 3 is an A-A cross-sectional view of a three-dimensional capacitance tomography (ECVT) device for flow in a cryogenic fluid venturi tube according to the present invention.

4 is a cross-sectional view of a B-B cross-sectional view of a three-dimensional capacitance tomography (ECVT) device for flow in a cryogenic fluid venturi tube according to the present invention.

5 is an axonometric view of a three-dimensional capacitance tomography (ECVT) device for flow in a cryogenic fluid venturi tube according to the present invention, the venturi tube.

6 is an axonometric view of a three-dimensional capacitance tomography (ECVT) device for flow in a cryogenic fluid venturi tube according to the present invention, and an electrode cover.

FIG. 7 is an axonometric view of a ring-shaped connection shell of a three-dimensional capacitance tomography (ECVT) device for flow in a cryogenic fluid venturi tube according to the present invention.

8 is an axonometric view of a metal casing of a three-dimensional capacitance tomography (ECVT) device for flow in a cryogenic fluid venturi according to the present invention.

FIG. 9 is an axonometric view of a conductor rod of a three-dimensional capacitance tomography (ECVT) device for flow in a cryogenic fluid venturi tube according to the present invention.

In the figure: 1. Conductor rod; 2. Metal shell; 3. Venturi tube; 4. Electrode sheet sleeve; 5. Electrode sheet; 6. Ring-shaped connection shell; 7. Connection groove; Connection boss; 10, positioning boss; 11, positioning groove; 12, wire hole; 13, wire groove; 14, metal hemisphere; 15, metal conductor; 16, insulating material; 17, elastic metal sheet.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings.

As shown in Figures 1-8, a three-dimensional capacitance tomography imaging device for flow in a cryogenic fluid venturi tube, which includes a conductor rod 1, a metal casing 2, a venturi tube 3, an electrode sheet sleeve 4, an electrode sheet 5, a ring-shaped connection shell 6;

There are two electrode sheet sleeves 4, and the two electrode sheet sleeves 4 form a complete electrode sheet device. The inner surface of the electrode sheet device is matched with the outer surface of the Venturi tube 3, and is fixed against the outer surface of the Venturi tube 3; The sheet sleeve 4 is provided with a plurality of pole piece grooves, the shape of the pole piece grooves is matched with the electrode piece 5, and the electrode piece 5 is installed in close contact with the pole piece grooves of the electrode piece sleeve 4;

There are two annular connecting shells 6, and the two annular connecting shells 6 form a complete connecting shell device; the peripheral circular diameter at both ends of the electrode sheet device is equal to the outer diameter of the connecting shell device, and the two ends of the electrode sheet device are respectively and The two ends of the connecting shell device are fixedly connected; the annular connecting shell 6 is provided with a plurality of metal hemispheres 14; the electrode sheets 5 on the electrode sheet sleeve 4 are welded with the metal hemispheres 14 by wires;

The metal shell 2 is provided with a plurality of semi-cylindrical grooves which are matched with the metal hemisphere 14. The inner diameter of the metal shell 2 is equal to the outer diameter of the connecting shell device. The shell 2 is provided with a circular hole for installing the conductor rod 1;

The inside of the conductor rod 1 is a metal conductor 15, the outer periphery is covered with an insulating material 16, and an elastic metal sheet 17 is provided at the bottom. The diameter of the semi-cylindrical slot; the conductor rod 1 is inserted through the circular hole to make the bottom elastic metal sheet 17 contact the metal hemisphere 14 on the annular connection shell 6, and the conductor rod 1 and the circular hole on the metal shell 2 are glued or welded; metal Both ends of the shell 2 are connecting flanges and have sealing grooves.

In a specific embodiment of the present invention, the electrode sheet device completely wraps the outer surface of the venturi tube 3; the structure of the two electrode sheet sleeves 4 constituting the electrode sheet device is the same; the two electrode sheet sleeves 4 pass through its two ends The provided connecting bosses 9 with barbs are connected with the connecting grooves 7 .

In a specific embodiment of the present invention, the connection shell device is cylindrical; the metal hemispheres 14 on it are evenly distributed in the circumferential direction, and the number is equal to that of the electrode sheets 5, and they are connected one by one; The two annular connecting shells 6 have the same shape; the connecting shell device is fixed in cooperation with the positioning boss 10 provided on the electrode sheet device through the positioning groove 11 provided thereon.

In a specific embodiment of the present invention, the shape of the electrode sheet 5 is the same as that of the electrode sheet grooves 8 of the electrode sheet sleeve 4, and the number is equal; Arrange multiple layers; the number of electrode sheet grooves in each layer is the same and evenly distributed along the circumferential direction, the electrode sheet grooves of two adjacent layers are staggered, the outer surface area of the electrode sheet is larger than the inner surface, and the electrode sheet sleeve 4 is embedded in the electrode sheet groove Fitted and fixed with Venturi 3.

In a specific embodiment of the present invention, the metal hemisphere 14 is fixed on the annular connecting shell 6 by screws.

In a specific embodiment of the present invention, the electrode sheet sleeve 4 is provided with a wire groove for fixing the wire; the annular connection shell 6 is provided with a wire hole for the wire to pass through and a wire groove for the wire to be fixed; the ring The wire hole on the annular connection shell 6 is aligned with the end of the wire groove of the electrode sleeve 4; The wire groove is welded with the metal hemisphere 14 .

In a specific embodiment of the present invention, the radius of the metal hemisphere 14 is the same as the radius of the semi-cylindrical groove on the inner surface of the metal shell 2 , and the outer diameter of the annular connecting shell 6 is the same as the inner diameter of the metal shell 2 .

In a specific embodiment of the present invention, the outer diameter of the insulating shell of the conductor rod 1 is the same as the diameter of the circular hole on the metal shell 2 and is larger than the diameter of the metal hemisphere 14 on the annular connecting shell 6, and the conductor rod 1 is inserted into the circular hole tightly. Fitted with the circular hole, the elastic metal sheet 17 at the end of the conductor rod 1 is in contact with the metal hemisphere 14 on the annular connection shell 6 .

In a specific embodiment of the present invention, the Venturi tube 3 , the electrode sheet sleeve 4 , and the annular connection shell 6 are made of insulating material 16 .

In a specific embodiment of the present invention, a structure of sixteen pole pieces is used, the electrode pieces are divided into four layers, each of which is four, the venturi tube, the pole piece cover, and the annular connecting shell are made of phenolic resin, and the electrode pieces are made of phenolic resin. The metal hemisphere on the sheet, the ring-shaped connection shell, and the metal rod in the conductor rod are made of oxygen-free copper, and the outer insulating material of the conductor rod is made of phenolic resin. The electrode sheet is embedded in the electrode sheet sleeve and the electrode sheet sleeve is fixed with the Venturi tube, and then each electrode sheet is welded with a wire with surrounding metal shielding. After fixing the wires along the wire groove, each wire passes through the wire hole on the annular connection shell. After the ring-shaped connection shell is fixed on the electrode cover, the wires going out from the wire holes are fixed along the wire grooves. The metal conductor hemispheres are fixed by screws and then welded to the wires. The conductor rod is first fixed on the metal shell by low temperature glue, the whole metal shell is sleeved into the annular connection shell, and is positioned by the elastic metal sheet symmetrically arranged at the end of the conductor rod. In actual work, it is necessary to pass cold gaseous working medium such as cold nitrogen and cold oxygen to complete the pre-cooling of the device to prevent brittle cracking caused by sudden cooling. In actual operation, first use the voltage U to excite one of the pole pieces, and the other pole pieces maintain 0 potential. At this time, measure the capacitance between the excitation pole piece and the other pole piece pairs, and then use the voltage U to excite the other pole piece. , measure the capacitance between it and other 0-potential pole pieces, and so on. It should be noted that the capacitance measurement is not repeated between the combination of the two pole pieces. The sensor from the acquisition of capacitance to the inversion into an image is called one operation. Therefore, for a 16-electrode ECT sensor, each operation can obtain 120 capacitance measurements for subsequent inversion imaging. Metal enclosures need to remain grounded.

The above-mentioned specific embodiments further describe in detail the structure, technical solution and operation mode of the device of the present invention in actual use. It should be understood that the above are only specific embodiments of the present invention, and are not used for The present invention is limited, but within the spirit and principle of the present invention, any modification, equivalent replacement, improvement, etc., should be included within the protection scope of the present invention.

Claims (10)

1. A three-dimensional capacitance tomography imaging device flowing in a cryogenic fluid Venturi tube is characterized in that comprising a conductor rod (1), a metal casing (2), a Venturi tube (3), an electrode sheet cover (4), an electrode sheet ( 5), annular connection shell (6); There are two electrode sheet sleeves (4), and the two electrode sheet sleeves (4) form a complete electrode sheet device. The inner surface of the electrode sheet device is matched with the outer surface of the venturi tube (3) and is closely attached to the venturi tube. (3) The outer surface is fixed; the electrode cover (4) is provided with a plurality of pole piece grooves, the shape of the pole piece grooves is matched with the electrode piece (5), and the electrode piece (5) is close to the pole of the electrode piece cover (4). slot installation; There are two annular connecting shells (6), and the two annular connecting shells (6) form a complete connecting shell device; the peripheral circular diameters at both ends of the electrode sheet device are equal to the outer diameter of the connecting shell device, and the diameter of the electrode sheet device is equal to the outer diameter of the connecting shell device. The two ends are respectively fixedly connected with the two ends of the connecting shell device; the annular connecting shell (6) is provided with a plurality of metal hemispheres; the electrode sheets (5) on the electrode sheet sleeve (4) are welded with the metal hemispheres through wires ; The metal shell (2) is provided with a plurality of semi-cylindrical grooves that cooperate with the metal hemispheres, the inner diameter of the metal shell (2) is equal to the outer diameter of the connecting shell device, and the metal shell (2) is sleeved into the circumference of the connecting shell device through the semi-cylindrical grooves. The metal casing (2) is provided with a circular hole for installing the conductor rod (1); The inside of the conductor rod (1) is a metal conductor, the outer periphery is covered with insulating material, and the bottom is provided with an elastic metal sheet. The diameter of the semi-cylindrical groove on (2); the conductor rod (1) is inserted through the circular hole so that the bottom elastic metal sheet is in contact with the metal hemisphere on the annular connection shell (6), and the conductor rod (1) is connected to the metal shell (2). The round hole of the metal shell (2) is glued or welded; both ends of the metal shell (2) are connecting flanges and are provided with sealing grooves. 2. The flow three-dimensional capacitance tomography imaging device in a cryogenic fluid Venturi tube as claimed in claim 1, wherein the electrode sheet device completely wraps the outer surface of the Venturi tube (3); The electrode sheet sleeves (4) have the same structure; the two electrode sheet sleeves (4) are matched and connected through the connection bosses with barbs and the connection grooves provided at both ends thereof. 3. The three-dimensional capacitance tomography device for flow in a cryogenic fluid venturi tube as claimed in claim 1, wherein the connecting shell device is cylindrical; the metal hemispheres on it are evenly distributed in the circumferential direction, and the number is the same as that of the electrode sheets. (5) The number is equal, and they are connected one by one; the two annular connecting shells (6) that constitute the connecting shell device have the same shape; fixed. 4. The flow three-dimensional capacitance tomography device in a low-temperature fluid venturi tube as claimed in claim 1, wherein the electrode sheet (5) has the same shape as the electrode sheet groove of the electrode sheet sleeve (4), and the electrode sheet has the same shape as the electrode sheet groove of the electrode sheet sleeve (4). The grooves are arranged in multiple layers along the axial direction of the electrode sheet sleeve (4); the number of electrode sheet grooves in each layer is the same, and evenly distributed along the circumferential direction, the electrode sheet grooves of two adjacent layers are staggered, and the outer surface area of the electrode sheet is larger than the inner surface area. , and the electrode sheet sleeve (4) and the venturi tube (3) are fitted and fixed after being embedded in the electrode sheet groove. 5 . The three-dimensional capacitance tomography device for flow in a cryogenic fluid venturi tube according to claim 1 , wherein the metal hemisphere is fixed on the annular connecting shell ( 6 ) by screws. 6 . 6. The flow three-dimensional capacitance tomography device in a cryogenic fluid Venturi tube as claimed in claim 1, wherein the electrode sheet sleeve (4) is provided with a wire groove for wire fixing; the annular connection shell (6) ) is provided with a wire hole for the wire to pass through and a wire groove for the wire to be fixed; the wire hole on the annular connection shell (6) is aligned with the end of the wire groove of the electrode sleeve (4); the wire passes through the electrode sleeve (4) The wire groove on the annular connecting shell (6) is passed through the wire hole on the annular connecting shell (6) and welded with the metal hemisphere through the wire groove on the annular connecting shell (6). 7. The three-dimensional capacitance tomography device for flow in a cryogenic fluid venturi tube as claimed in claim 1, wherein the radius of the metal hemisphere is the same as the radius of the semi-cylindrical groove on the inner surface of the metal shell (2), and the annular connection shell (6) The outer diameter is the same as the inner diameter of the metal shell (2). 8. The three-dimensional capacitance tomography imaging device for flow in a cryogenic fluid venturi tube as claimed in claim 1, wherein the outer diameter of the insulating casing of the conductor rod (1) is the same as the diameter of the circular hole on the metal casing (2) and is The diameter of the metal hemisphere on the annular connecting shell (6) is larger than that of the annular connecting shell (6). 9. The three-dimensional capacitance tomography imaging device for flow in a cryogenic fluid venturi tube as claimed in claim 1, wherein the venturi tube (3), the electrode sheet cover (4), the annular connection shell (6) The material is insulating material. 10. A capacitance measurement method of a flow three-dimensional capacitance tomography device in a cryogenic fluid venturi tube according to claim 1, characterized in that: The metal shell is kept grounded, and the pre-cooled gaseous working medium is used to pre-cool the device to prevent brittle cracking caused by sudden cooling; In actual operation, first use the voltage U to excite one of the pole pieces, and the other pole pieces maintain 0 potential, respectively measure the capacitance value between the excitation pole piece and the other pole piece pairs, and then use the voltage U to excite the other pole piece, and measure the The capacitance value between it and other 0-potential pole pieces, and so on until the capacitance measurement between all pole pieces is completed; the capacitance measurement is not repeated between the combination of two pole pieces; the collected capacitance value is used for subsequent inversion imaging.

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